Karl Fischer's (hereinafter abbreviated as "KF") titration has been used widely for measurement of a water content in various samples. With the latest development of electronics, operations of apparatus for water content measurement have been made easier and simpler with increased accuracy and precision. KF titration is roughly classified into volumetric titration and coulometric titration which are carried out in different apparatus with KF reagents having different compositions suited for the respective method and apparatus.
The anolyte for KF coulometric titration (KF reagent) is composed of an iodide ion, sulfur dioxide, a basic compound, and a solvent.
KF titration is based on the following KF reaction: EQU I.sub.2 +SO.sub.2 +H.sub.2 O+3Base.fwdarw.2Base.HI+Base.SO.sub.3( 1)
(Base: basic compound)
In the anolyte for coulometric titration, iodine is supplied through electrode reaction represented by the formula: EQU 2I.sup.- .fwdarw.I.sub.2 +2e (2)
If water is present, iodine produced by electrolysis of formula (2) is consumed according to formula (1). On exhaustion of water, iodine becomes excessive. The end point of titration is obtained by detecting the excess of iodine. The quantity of electricity in formula (2) which has been used up to the end point is proportional to the water content.
The KF reaction of formula (1) is dependent on the pH of the system and is known to show a higher rate at a higher pH value within a certain range of pH as reported in J. C. Verhoef and E. Barendrectt, J. Electroanal. Chem., Vol. 71, p.305 (1976).
The basic compound, one of components constituting the KF reagent, is required for maintaining pH within the above-mentioned certain range in which the KF reaction may proceed. To this effect, the single use of pyridine, imidazole (U.S. Pat. No. 4,378,972), triethanolamine, or morpholine (U.S. Pat. Nos. 4,429,028 and 4,740,471) has been proposed to date. However, when pyridine is used alone in a conventional electrolytic solution, the anolyte has a low pH so that the KF reaction tends to be slow, requiring slightly somewhat greater time for analysis. In using imidazole alone, the anolyte has a high pH so that the KF reaction tends to rapidly proceed, resulting in over-titration. Triethanolamine or morpholine provides too high basicity, failing to allow a normal KF reaction to proceed.
The inventors previously proposed an anolyte for KF coulometric titration which contains a mixed basic compound system comprising pyridine derivatives free from a pyridine odor as disclosed in U.S. Pat. No. 4,720,464. This anolyte achieves a higher reaction rate than in a pyridine-containing anolyte and is less causative of over-titration than with an imidazole-containing anolyte. The mixed basic compound system proposed is a mixture of an aminopyridine derivative, e.g., 4-dimethylaminopyridine and 2-methylaminopyridine, and a pyridine derivative having two pyridine nuclei linked via an alkylene group, e.g., 1,3-di(4-pyridyl)propane and 1,3-di(2-pyridyl)propane.
However, the reduction in time of analysis achieved with the above-described electrolytic solution is still insufficient. Moreover, this electrolytic solution is usable only as an anolyte, still needing further improvement so as to have such a broadened application both as an anolyte and as a catholyte, as has been recently advanced in the art.
On the other hand, chlorine-containing solvents used in most conventional titration reagents, such as carbon tetrachloride and chloroform, are not deemed suitable from the standpoint of environmental conservation. It has therefore been demanded to develop a titration reagent which is free from chlorine and still exhibits satisfactory performance.